ssh tunneling and "channel 2: open failed: administratively prohibited: open failed"

[Jonathan Underwood]

Hi,

I'm having a spot of bother getting an ssh tunnel to work. Here's the situation:

I am trying to connect to machine C via a tunnel to machine B from
machine A, with machine A being the local machine.

Machine C is behind a firewall, and so direct ssh from A to C isn't possible.

At the command line, once ssh'd into machine B from machine A, I can
ssh from machine B to machine A.

So, on machine A (local machine) I run in one terminal (Terminal 1):

ssh  -R 8888:C:22 username at B cat -

which asks me for my password on machine B, which I duly enter.

Then, on machine A, in a second terminal (Terminal 2) I run

ssh -p 8888 localhost

On the terminal (Terminal 1) which is running the ssh tunnel I get this message:
channel 2: open failed: administratively prohibited: open failed

And on Terminal 2 I get
ssh_exchange_identification: Connection closed by remote host

Oddly, at no point have I been asked to enter login details for machine C.

I've ensured that port 22 is allowing incoming ssh connections on
machine C. I've tried turning off SElinux on machine C. Machnine B is
not under my control, so there's nothing much I can do there -- it is
a SunOS machine and the ssh client does support the options for port
forwarding (-L and -R).

Getting desperate - what am I doing wrong? :)

TIA
Jonathan.